The adhesive layer of an adhesive sheet is generally a single layer if it contains elastic microspheres. Such an adhesive sheet is produced by applying a coating liquid containing elastic microspheres and a tacky polymer directly onto the surface of a substrate and drying the applied coating liquid to form a single layer adhesive film as disclosed in JP-A-6-287525. When such an adhesive sheet is press bonded onto an adherent, raised portions comprising the elastic microspheres and the adhesive layer are in spot contact with the adherent, and passages which can communicate with the atmosphere are formed between the adhesive layer and the adherent. Bubbles which have been trapped between the adhesive layer and the adherent can be easily removed when they are forced to escape by the application of a pressure from the back face of the support towards the adherent in the presence of those passages.
However, the adhesive layer tends to maintain the above spot contact state after the adhesion of the adhesive sheet to the adherent (finish pressing), and therefore the peeling adhesion strength is low. For example, a DINOC.TM. brand film (commercially available from Sumitomo 3M Co. Ltd. of Tokyo, Japan) which is an example of a decorative sheet having an adhesive layer containing no elastic microspheres has a peel adhesion strength (90 degree peeling strength) of at least about 2 kgf/25 mm (when a bonderized steel plate is used as the adherent), while an adhesive sheet having a single layer adhesive film containing elastic microspheres has the peeling adhesion strength of at most 1.0 to 1.5 kgf/25 mm against the same steel plate.
That is, conventional adhesive sheets containing elastic microspheres are suitable as repositionable sheets, but they are unsatisfactory as decorative sheets which require permanent bonding.
Further, water or vapor (or moisture) penetrates into the interface between the adhesive layer and adherent through the passages and thus an adhesion strength decreases, that is, the water-resistant adhesion strength decreases, if the passages remain after the completion of adhering when the adhesive sheet is used for decorating bath rooms, changing rooms, and so on.
One of the reasons for the continuation of spot contact, that is, the passages communicating with the atmosphere, may be that the tacky polymer layer covering the elastic microspheres is in a single layer state having a small thickness in comparison to the diameter of the microspheres, has small flow during the press adhering of the sheet to the adherent and small flow over time (namely, cold flow), and therefore the contact areas of the adhesive layer do not increase readily.
JP-A-6-322325 discloses an example of adhesive sheets having a dual adhesive layer. The used adhesive layer is prepared by laminating a primer layer containing hollow microparticles and a top adhesive layer on the primer layer. The primer layer functions to compensate surface irregularity of the adherent such as a corrugated board, etc. after the adhesive sheet is adhered to the adherent, and thus good adhesion of the sheet to the irregular surface is achieved. However, the contact between the adhesive sheet and adherent is plane contact since the top surface of the adhesive layer has no projections, and therefore the bubble escapability is not good.
JP-U-5-72946 discloses an adhesive sheet or tape having a dual adhesive layer containing elastic microspheres. The disclosed adhesive tape or sheet comprises a substrate, an adhesive layer containing elastic microspheres, and a layer of viscoelastic materials having a thickness of 20 pm inserted between the substrate and adhesive layer. This structure improves the adhesion of the tape or sheet to adherents having irregular surfaces such as a corrugated board, etc. and achieves repositioning properties of the tape or sheet while preventing spontaneous peeling. However, large cold flow cannot be expected with this structure, and the contact areas of adhesive layer do not readily increase.